Can containers can be roughly classified according to their parts and configurations as either two-piece cans each composed of a main body and a top lid or three-piece cans each composed of a main body, top and bottom lids. In such a three-piece can, its main body is connected by a process such as soldering, resin bonding, welding or the like.
Recently, a demand has been increased for designed cans having more three-dimensional shapes than simply cylindrical cans, from the viewpoint of improving the design of cans. These circumstances are presented in, for example, a journal "THE CANMAKER February 1996, p32-37".
These designed cans are mainly manufactured as three-piece cans, formed into a cylindrical shape, connected and then formed into an objective shape such as a barrel shape by imparting strain in the circumferential direction to a cylindrical connected main body with the use of a delicate split tool, hydrostatic press or other technique.
The designed cans manufactured by such techniques are called as modified three-piece cans, and require being superior in the following properties to those of conventional three-piece cans.
(1) That no fracture occurs in a secondary forming (this term means a working for imparting design to the can after forming of a cylinder; hereinafter it will have the same meaning),
(2) that no defective appearance occurs in the secondary forming,
(3) that less decrease occurs in can height in the secondary forming, are required. Major fracture forms in the secondary forming include fractures in the vicinity of welded joint and fractures of main body unit, and major defective appearances in the secondary forming include rough surfaces and stretcher strain. When the can height decreases in the secondary forming, the can capacities of finished products or the yields of materials are hardly ensured. The higher a r-value is, the more the can height decreases.
In addition, in consideration of recent demands to reduce the thickness of materials for cost-reduction,
(4) that a material strength (hardness) is high,
(5) that the yield strength (YS) of a material is not excessively high, are also required. If the material strength (hardness) is low, the can body strength cannot be ensured, whereas if the yield strength (YS) of the material is excessively high, springback increases and weldability is deteriorated due to decreasing the roundness of the cylinder or variations in lap allowance.
Incidentally, manufacturing processes of can steel sheets have been roughly classified into;
(i) a manufacturing process of subjecting a low-C steel containing C:
about 0.01 to 0.10%, preferably 0.03% or more, to cold rolling and then to box annealing,
(ii) a manufacturing process of subjecting a low-C steel to cold-rolling and then to continuous annealing.
(iii) a manufacturing process of subjecting a material steel (IF steel) to cold-rolling and then to continuous annealing, which material steel is obtained by adding a strong solute-C-stabilizing element such as Ti or Nb to an extra low-C steel containing C: less than 0.01%.
According to the process (i) of subjecting a low-C steel to box annealing, however, workability in secondary forming generally tends to be satisfactory, but the r-value can hardly be decreased, so that the decrease in can height cannot be avoided. According to this process, crystal grains are liable to become coarse and rough surfaces are somewhat liable to occur, inviting defective appearance. In addition, the steel become soft and the strength is hardly ensured, whereas if the steel is subjected to a generally-employed secondary rolling, it becomes hard, inviting an excessively high YS.
On the other hand, according to the process (ii) of subjecting a low-C steel to continuous annealing, the r-value can be decreased yet insufficiently as compared with the process of subjecting a low-C steel to box annealing, and the crystal grains become fine and hence the steel is facilitated to prevent rough surfaces and to ensure strength (hardness). However, the workability is insufficient, and fractures in particular in the vicinity of welded joint are liable to occur in the secondary forming. In addition, non-aging properties cannot be achieved and stretcher strain is liable to occur according to this process.
The process (iii) of subjecting an IF steel to continuous annealing generally provides excellent non-aging property, but allows the crystal grains to become coarse and hence are most disadvantageous for preventing rough surfaces and the highest in the r-value. These problems may provably be resolved by a process of conducting annealing in an imperfect manner, but sufficient workability for secondary forming can hardly be obtained.
As described above, according to the conventional processes, it is difficult to reduce the r-value less than 1.0 and to minimize the decrease in can height, and, in general, prevention of rough surfaces and ensuring of the secondary forming workability/non-aging property are hardly compatible.
Japanese Unexamined Patent Publication No. 1-116030 discloses a technique of subjecting a substantially low-C steel containing C: 0.10% or less to continuous annealing at a recrystallization temperature or higher and 800.degree. C. or lower, and then to box annealing at a temperature ranging from 300.degree. C. to 700.degree. C. This technique provides a steel sheet for easy open can lid containing fine grains of grain size number #9 or more (corresponding to a mean grain diameter of 17.6 .mu.m or less), being non-aging properties as is not aged by bake-coating of the lid, and being excellent in, for example, easy-open property. Even according to this technique, however, the r-value becomes 1.0 or more, and its secondary forming workability, hardness and rough-surface resistance do not meet the levels required in modified three-piece cans to which the present invention is directed.
It is an object of the present invention to provide a can steel sheet which can solve the problems of the conventional technologies and meets the workability, appearance property after working and high yield meeting even complicated requirements in can designs, and a method of manufacturing the same. It is another object of the present invention to provide a can steel sheet which effectively prevent the formation of surface defects due to alumina and other cluster inclusions, is satisfactory in surface appearance such as the aesthetics of appearance and defect-free property, and excellent in formability in welded joint, and a method of manufacturing the same.